The development of simultaneous resistance to multiple structurally unrelated drugs is a major impediment to cancer chemotherapy. (Roepe, Curr. Pharm. Des. 6:241, 2000). Multidrug resistance is associated with specific DNA sequences termed the multidrug resistance locus (mdr1) (Roepe, Curr. Pharm. Des. 6:241, 2000). Increased expression and amplification of mdr1 sequences have been reported in numerous multidrug-resistant sublines of human leukemia and ovarian carcinoma cells. Overexpression of the mdr1 gene product (P-glycoprotein) reportedly is a feature of mammalian cells displaying resistance to multiple anticancer drugs and has been postulated to mediate resistance. Multidrug resistance correlates with amplification of two related DNA sequences, designated mdr1 and mdr2 (mdr2 has been referred to by others as mdr3). These sequences were isolated through homology with the Chinese hamster mdr gene. mdr1 encodes a 4.5-kb mRNA and reportedly was amplified or overexpressed in all multidrug-resistant human cell lines analyzed. (Hoffmeyer, S., et al., Proc. Natl. Acad. Sci. USA 97:3473).
The mdr1 gene product P-glycoprotein extrudes a variety of drugs across the plasma membrane. The homologous mdr3 P-glycoprotein is required for phosphatidylcholine secretion into bile. By stable transfection of epithelial cells, mdr1 and mdr3 were localized in the apical membrane. The mdr1 gene product (P-glycoprotein) is the apical membrane protein responsible for the renal secretion of digoxin. This agent has a low therapeutic index and a relatively large and diverse group of coadministered drugs are reported to interact with digoxin, for example, quinidine, verapamil, amiodarone, spironolactone, and cyclosporin, frequently leading to its toxic accumulation (Ueda, et al., Proc. Nat. Acad. Sci. 84:3004–3008, 1987). Since digoxin is a prototype for endogenous digitalis-like glycosides, endogenous digitalis-like glycosides may be the natural substrates for P-glycoprotein. (de Lannoy, I. A. M., et al., Biochem. Biophys. Res. Commun. 189:551–557, 1992).
Increased levels of P-glycoprotein reportedly occur in some osteosarcomas. Baldini et al. (Baldini, et al., New Eng. J. Med. 333:1380–1385, 1995) investigated the relationship between P-glycoprotein status and outcome in 92 patients with high-grade osteosarcoma of the extremities who were treated with surgery and chemotherapy. The presence of increased levels of P-glycoprotein in the osteosarcoma reportedly were significantly associated with a decreased probability of remaining event-free after diagnosis. In a multivariate analysis, P-glycoprotein status and the extent of tumor necrosis after preoperative chemotherapy were independent predictors of clinical outcome. HIV-1 protease inhibitors are potent agents in the therapy of HIV-1 infection. However, limited oral absorption and variable tissue distribution complicate their use. Kim et al. (Kim, et al., J. Clin. Invest. 101:289–294, 1998) reported that P-glycoprotein-1 is involved in the transport of three of these protease inhibitors in vitro. After oral administration, plasma concentrations were elevated 2- to 5-fold in mdr1 α −/− mice carrying the disrupted mdr1 α gene, and with intravenous administration, brain concentrations were elevated 7- to 36-fold. The literature also suggests that P-glycoprotein limits the oral bioavailability and penetration of these agents into the brain (Schinkel, et al., Cell 77:491–502, 1994).
Human solid tumors are considerably less well oxygenated than normal tissues (Brown, Mol. Med. Today, 6:157, 2000). This leads resistance to radiotherapy and anticancer chemotherapy, as well as predisposing to increased tumor metastases, the mechanism(s) of which are not known at present. The microenvironment of rapidly growing tumors is associated with increased energy demand and diminished vascular supply, resulting in concentric areas of cellular hypoxia (Brown, Mol. Med. Today, 6:157, 2000). A number of hypoxia-responsive genes have been associated with growing solid tumors.
In view of the foregoing, a need exists to better understand the mechanism underlying multidrug resistance and cancer genesis and to develop targeted therapeutic agents for treating these and other disorders.